Automatic transmission vehicles are generally provided with an operator manipulated range selector, whereby the operator selects one of a number of transmission ranges including Park, Reverse, Neutral, Drive and one or more Manual Lo ranges. The Manual Lo ranges are used to limit the uppermost available speed ratio (gear), and in most cases, to provide engine braking. In conventional practice, the range selector is physically coupled to the transmission through a suitable mechanical or electro-mechanical linkage, where it operates to position a linearly displaceable hydraulic valve, commonly referred to as a Manual Valve. Although linearly displaceable, the Manual Valve is provided with a number of discrete positions corresponding to the various positions of the range selector and operates to develop fluid range or supply pressures within a hydraulic valve body of the transmission for enabling the range selected by the operator.
In normal usage, the term "range pressure" refers to the working fluid pressure distributed to the various hydraulic control elements (shift valves, e.g.) of the transmission. As noted above, this is conventionally accomplished with the Manual Valve based on the position of the range selector. For example, if the range selector is positioned to the Reverse sector, the Reverse range pressure circuit of the transmission is pressurized to engage the Reverse clutch or brake. If the range selector is positioned to the Drive sector, the Drive range pressure circuit is pressurized to enable engagement of any of the forward drive ranges; in this case the engagement of a selected clutch or brake is carried out by one or more shift valves to which the Drive range pressure is supplied. The range pressure within a transmission may therefore be considered as a range-dependent supply pressure to be used for the engagement of selected fluid operated clutches or brakes. This is in contrast to the clutch apply pressure, which varies with the operation of the various hydraulic pressure control elements.
Although simple in concept, the above-described arrangement presents various drawbacks pertaining to installation and packaging. The linkage, which is factory adjusted at the time of installation to ensure proper alignment among the range selector, Manual Valve and a range indicator, is subject to subsequent misadjustment by untrained service personnel. Packaging concerns pertain primarily to the routing of the linkage mechanism to the transmission.
More significantly, however, the conventional range selector apparatus is custom designed and limits the flexibility of transmission control. For example, if the manufacturer wishes to provide engine braking in certain ranges for a particular application, the Manual Valve must be specifically tailored to route fluid range pressure to the appropriate hydraulic elements. Each application requires a custom design and an application specific set of parts for installation into the transmission.
In addition, it may be desirable to automatically alter the fluid range pressure routing at will, depending on the vehicle operating conditions. For example, it would be desirable to selectively utilize engine braking or to override the manual selector position under certain conditions.